The present invention relates to a phase synchronizing circuit for the demodulation of multi-phase PSK signals, and more particularly, to the circuit of this type having a broadened capture frequency range without causing false capture.
Various attempts have been made to broaden the frequency range within which the recovered carrier wave for PSK demodulation is brought to a phase-synchronized state. Such frequency range is often called a capture frequency range. One of the proposals is disclosed in Japanese Pat. No. 837,502 (Japanese Patent Publication No. 51-10752) invented by Yoshio Matsuo.
The circuit proposed in this Japanese patent employs a low frequency oscillator coupled to the phase synchronizing loop, which has a phase comparator and a voltage controlled oscillator (VCO). The output of the low frequency oscillator is superimposed on the control voltage supplied through the loop and applied to the VCO for frequency sweep. So long as phase synchronization is not established, the loop exhibits an appreciable impedance and keeps the low frequency oscillator on so that the VCO continues the frequency sweep. Once the phase synchronization is achieved, the loop turns low in impedance, causing the low frequency oscillator to stop oscillation, thereby stopping the frequency sweep by one VCO.
When this phase synchronizing circuit is employed in a demodulator circuit for an N-phase phase-modulated signal, the phase-synchronized state is established not only at the desired center frequency but also at frequencies deviated by fc/n therefrom (n being a positive integer and fc representing the clock frequency), causing the false capture. Once the phase-synchronized state is established, whether it is a desired or false capture, the oscillation at the low frequency oscillator is stopped due to the lowered loop impedance. Therefore, if the phase-synchronized state is due to a false capture, the recovery of the desired phase synchronization is rendered impossible. The correct demodulation of the N-phase phase-modulated signal is accordingly rendered impossible.